Justice is a concept involving the fair and moral treatment of all persons, especially in law. It is often seen as the continued effort to do what is "right." In most of all cases what one regards as "right" is determined by consulting the majority, employing logic, or engaging in mysticism. If a person lives under a certain set law in a certain country, justice is considered making the person follow the law and be punished if not.
Classically, justice was the ability to recognize one's debts and pay them. It was a virtue that encompassed an unwillingness to lie or steal. It was the basis for the code duello. In this view, justice is the opposite of the vice of venality.
Justice (in both senses) is part of the debate regarding moral relativism and moral absolutism: Is there an "absolute standard" of justice, under which all behavior should be judged, or is it acceptable for justice to have different meanings in different societies? Some cultures, for instance, see punishments such as the death penalty as being appropriate, while others decry such acts as crimes against humanity.
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injustice, when it comes into being on a sufficient scale, is mightier, freer, and more masterful than justice; and, as I have said from the beginning, the just is the advantage of the stronger, and the unjust is what is profitable and advantageous for oneself.
But the injustice of the second part of the statement implies that the "other" in the first part is not the ruling tyrant, but the ruled many.
So, it is clear that the praising of injustice from the ruler's perspective rests upon a standard of justice that is found to be the case from the ruled's perspective and therefore, the ruler never really escapes the standards of justice and injustice as Thrasymachus would want us to believe.